In a well completion, a string of casing liner, or pipe is set in a borehole and a material such as cement is forced into the annulus between the casing and the borehole primarily to separate oil and gas producing formations from each other and from water bearing strata. Obviously, if the cementing fails to provide a separation of one zone from another, then fluids under pressure from one zone may be able to migrate and contaminate an otherwise productive nearby zone. Migration of water, in particular, produces undesirable water cutting of a producing zone and possibly can make a well non-commercial.
It is a problem to obtain an accurate picture of conditions behind a casing because of the difficulty of propagating signals through the casing wall. Various prior proposals to determine the separation effectiveness, (i.e., the blocking or sealing characteristics) of the cement behind the casing have not been entirely successful in clearly determining the effective presence of cement in the annulus between the casing and the formation. Further, it has not been possible to measure reliably the quality of the cement bond between the casing and the cement.
The mere presence or absence of cement in the annulus between the casing and formation is valuable information, however, this does not provide a complete picture of the cement conditions. While cement may be present in the annulus, channels or inadequate sealing may still permit fluid communication between adjacent formations.
Several prior developments for obtaining a measure of the quality of a cement bond relative to the casing have been disclosed in U.S. Pat. Nos. 3,291,274, 3,291,248 and 3,292,246. These systems generally utilize acoustical principles where an acoustical signal is transmitted between a transmitter and a receiver. The amplitude of the early arrival signal (this early arrival usually is the casing signal since the acoustical energy under average conditions generally travels faster in the casing than in the surrounding cement or formation) at the receiver is measured as a determination of the quality of the bond of cement to the casing. If a good bond existed, the casing signal would be expected to be attenuated because of the energy dissipated from the casing to the cement and surrounding formations, whereas if no bond or a poor bond existed the casing signal would be relatively unattenuated.
An additional technique for determining the quality of cement in the annulus between the casing and the formations is disclosed in U.S. Pat. No. 3,401,773. In this technique the amplitude of a reverberated early (casing) signal arrival is recorded and additionally, the total energy of a selected later portion of the sonic signal is obtained by integration to provide a second indication of the quality of the cement bond. Even in the absence of a weak casing arrival, the additional step of observing the total energy obtained by integrating a later portion of the signal in this manner can confirm the presence of cement in the casing-annulus-formation system.
It has been established that the energy content of the acoustical logging signals arriving at the receiver depends on other factors than the quality of the cement bond to the casing or the integrity of the cement column (sometimes called cement quality). The following factors were found to have substantial effect on signal arrivals, receiver sensitivity; the formation hardness; eccentering of the acoustical logging tool; the high temperature environment and the temperature variations in the well bore; type of casing; and the diameter of the borehole and casing as well as their shape or geometry.
A particular important shortcoming of previous cement evaluation logging systems which measure and rely only on the attenuation rate measurement is that this measurement is strongly affected by the microannulus which often appears between casing and the solid cement column after the cement has set up or solidified. These microseparations may occur either within the cement sheath itself, or at the interfaces of the cement column with the steel casing or formation. They may result from several factors including expansion and/or contraction of the casing due to either temperature or pressure cycles or borehole fluid changes or shock and vibration occurring during the well completion process or to shrinkage of the cement itself. Often these microseparations are small enough that the permeability of the annulus is not significantly modified, and the hydraulic seal offered by the cement column not impaired Microannuli do, however, severely reduce the acoustical attenuation rate measured by tools which determine the attenuation rate, and they cause the received amplitude to increase for tools which measure the peak amplitude only. Those measurements are thus not reliable measurements for use in estimating the hydraulic seal.
Currently the way in which cement evaluation is conducted is by the placing of some type of sonde tool inside the casing. This tool is passed through the casing from the bottom of well across the zone of interest. A sound wave passes through the casing, cement and formation and returns to a receiver. The quality of the cement job is then judged by examining the amplitude curve on the cement evaluation tool. This amplitude curve is read in millivolts with a dual scale normally being used of 0-20 mV and 0-100 mV. The voltage determined by the tool is a representation of attenuation rate, normally measured in decibels/ft. This attenuation rate is what is measured by the tool and the voltage is an electronic representation of it. The interpretation of this log is normally then done in the following method: the compressive strength of the cement slurry is determined experimentally, historically or a given value is used. This compressive strength is then converted into attenuation rate by use of a nomograph where a straight line is drawn from the compressive strength of the cement through the thickness of the casing to determine the attenuation rate. The attenuation rate is then converted into a voltage and this voltage is then considered to be a 100% bonded section. This method has been found to be most inadequate because data has been shown that proves no relationship exist between compressive strength and attenuation rate. (Jutten, J. J., Guilot, D., and Parceveaux, P. A., Society of Petroleum Engineers, Paper No. 16652-Revised).
U.S. Pat. No. 4,896,303 recognized the need to determine the acoustical impedance of the material believe well casing However, it only teaches a new apparatus and method which requires deriving a compensated attenuation rate signal which is representative of the attenuation of the received acoustical signal and a coupling signal representative of the attenuation of the received acoustical signal. As only a measurement of acoustical coupling is determined, only a relative measure of compressure strength can be generated. No new method of interpreting the acoustical data or deriving the attenuation rate is provided.
U.S. Pat. Nos. 4,893,285, and 4,757,479 issued to Masson et al. teach a method of measuring cement quality, whereby a Sonde transmits acoustical energy which excites the casing, annulus, and formation, and the ratio of the signals received by two longitudinally spaced receivers is examined. Peak amplitude of a portion of the acoustical casing signals are measured, and ratios of these peaks are studied to determine cement quality. No new method of interpreting the acoustical, data is provided.
U.S. Pat. No. 4,703,427 issued to Catala et al. teaches a method of obtaining ultrasonic pulse signals recorded by a cement evaluation tool and dividing the signals into time windows After normalization of the signal magnitudes, cement quality is determined by comparing the magnitudes with the measured acoustical impedances of various media. None of the prior work describes a method of using cement slurry density volume to generate an acoustical impedance map to compare, along with an attenuation rate map, to a voltage map to compare the theoretical value of acoustical impedance to a calculated acoustical impedance, to predict the quality of a material, such as cement, disposed behind the casing in a wellbore. There is therefore a need for such a method to provide an inexpensive, quick, and accurate determination of cement quality.